1. Field of the Invention
The present invention relates to light-shielding coatings and optical elements, in particular, a light-shielding coating that absorbs stray light and an optical element featuring this light-shielding coating. Stray light in optical parts such as lenses and prisms may cause flare and ghost.
2. Description of the Related Art
Cameras, microscopes, and other optical apparatuses usually incorporate optical members, such as lenses and prisms, and may have its optical performance deteriorated owing to flare and ghost caused by the reflection of incidence light on the surfaces and the inner walls of the optical members (hereinafter, referred to as “surface reflection” and “inner reflection”). A known example of countermeasures against surface and inner reflection is antireflection coatings, which are obtained by applying black paint to the lateral sides of optical members.
Japanese Patent Publication No. 55-30029 has disclosed an antireflection coating composed of either coal tar or coal-tar pitch and a polymer containing bromine atoms, iodine atoms, or other kind of halogen atoms. Not only does this antireflection coating reduce inner reflection but it reduces surface reflection by its surface unevenness brought about by silica fine particles.
Japanese Patent Laid-Open No. 7-82510 has disclosed a paint composition composed of inorganic black fine particles. This paint composition also prevents inner reflection.
As mentioned in Japanese Patent Publication No. 55-30029, coatings composed of organic compounds such as coal tar and coal-tar pitch can be prevented from surface reflection in an ordinary way of matting materials, specifically, by making their surfaces rough with silica fine particles. However, coatings produced for the prevention of inner reflection usually contain high-refractive-index fine particles; when further treated with silica fine particles for the prevention of surface reflection, such coatings may have a seriously decreased refractive index owing to the association of the fine particles for the prevention of inner reflection and those for the prevention of surface reflection.
The coating disclosed in Japanese Patent Laid-Open No. 7-82510, which contains inorganic black fine particles, can admittedly prevent inner reflection thanks to the high refractive index of the resin contained therein, but it cannot prevent surface reflection effectively.